A fuel cell system is a system in which a fuel gas and an oxidizing agent gas are supplied to a fuel cell via corresponding gas feed passages, and electricity is generated by using an electrochemical reaction of these gases in a fuel cell body.
Water is generated in the fuel cell body by the electrochemical reaction, and the water is discharged from the fuel cell body in a condition that the water is contained in a fuel-off gas (a fuel gas discharged from the fuel cell body) and an oxidizing-agent-off gas (an oxidizing agent gas discharged from the fuel cell body).
Therefore, if the ambient temperature of the system falls to the freezing point or below in a stop state of the fuel cell system, the moisture in the gases remaining in the valves, the piping, etc., arranged in the gas passages of the system, may condense and the component parts of the system may be frozen.
In such a case, there is a possibility that starting the fuel cell system is impossible even if an operation to start the fuel cell system is performed thereafter. Or, there is a possibility that, even if the fuel cell system can be started, it is difficult to perform the normal operation of the fuel cell system. In particular, the freezing of a gas supplying device, such as a pump, would make the supplying of fuel gas or oxidizing agent gas impossible, and considerable time would be needed to put the entire system into operation.
To avoid the problem, there has been proposed a method of removing residual water in which a liquid storing section is arranged at the inlet port or discharge port of a hydrogen pump so that the moisture does not easily remain in the pump chamber of the hydrogen pump, and the inside of the hydrogen pump is scavenged with dry hydrogen gas immediately before occurrence of a system stop. See Japanese Laid-Open Patent Application No. 2003-178782.
There has been also proposed a method of removing residual water in which dry hydrogen gas is supplied to a hydrogen pump at a predetermined timing in a system stop phase and the residual water is discharged, in order to prevent freezing of the hydrogen pump. See Japanese Laid-Open Patent Application No. 2005-158426.    Patent Document 1: Japanese Laid-Open Patent Application No. 2003-178782    Patent Document 2: Japanese Laid-Open Patent Application No. 2005-158426